This invention relates generally to airborne navigation systems and more particularly to a system for determining aircraft location, wind information and lateral steering command from distance measurements made from plural ground transponders.
The system to be described herein thus relates to those systems which precisely locate aircraft in flight by making use of existing transponder systems of the TACAN, VORTAC, VOR/DME or DME type, by having the aircraft measure distances to a number of ground stations in rapid succession and utilizing these measured distances to compute aircraft position. The use of TACAN, VORTAC, VOR/DME or DME ground and airborne units to determine location is attractive and has heretofore been exploited because of the number of ground stations available all over the world, and because DME type units are well developed and utilized in many of today's aircraft. However, little attention has been devoted to using this type of equipment for precision measurements of location, mostly because the DME equipment was initially designed to provide convenient enroute navigation information which required only nominal accuracy.
In the prior art, for example, there is a large number and variety of systems available by means of which an aircraft may be located with respect to multiple ground stations, as by measuring the phase difference between signals emanating from the ground stations and processing them in the computer to compute the aircraft location. In other instances, by measuring distances between the aircraft and ground stations, and processing these ranges in a computer, aircraft position has been obtainable. This type of system is discussed in a paper by Latham, in the publication "Navigation", Vol. 21, No. 2, 1974. The type of system therein described depends on prior storage of a limited number of available stations in the immediate service area and range gate prepositioning to reduce range measurement time. Such systems give rise to attendant ground station saturation problems and generally utilize a "track" mode to achieve the required accuracy. The system to be described is of this general type, since it involves interrogation of DME ground station transponders and utilizes replys from the ground stations to the aircraft.
To achieve a high degree of accuracy while utilizing existing navigation facilities, more than the minimum number (usually considered to be three to avoid ambiguity) of ground station measurements must be obtained, and an increase beyond this number of measurements yields greater accuracy. Such systems have been utilized by processing distance measurements from a plurality of ground DME stations (all within range) in a computer towards achieving maximum accuracy in the operating environment. In these types of systems sufficient redundant information permits the computer to estimate the error in each of the distance measurements and compensate for these errors to improve the position accuracy. Such an approach is disclosed in Potter et al., U.S. Pat. No. 3,659,085. Further known systems of this type are described in Miller U.S. Pat. No. 4,035,801 and Chisholm et al. U.S. Pat. No. 3,821,523. Chisholm et al. U.S. Pat. No. 3,821,523, for example, relates to a system utilizing a single DME unit in the aircraft of the agile tuning type which is presently available and causes the airborne DME unit to rapidly tune to a predetermined number of preselected DME ground stations and utilizes the distance measurements along with the coordinate location and elevation of the associated ground stations in a computer algorithm to obtain instant aircraft location. In the known prior art, however, a selected (preset) number of ground stations within a service area in which the aircraft is to be flown are set into the system control, and distance determinations are available for operation within the service area of the selected ground stations by causing the DME interrogator unit in the aircraft to tune in sequence to the preselected ground stations.